The present invention relates to a braking system, intended primarily for use in highway vehicles, in which a friction brake is associated in the conventional manner with each wheel of the vehicle and a hydrodynamic brake in provided on one axle, generally the rear axle, of the vehicle.
The hydrodynamic brake and the friction brakes may be controlled by means of separate respective operating levers, or the hydrodynamic brake may be controlled jointly with the friction brakes by means of a single operating lever.
One common problem arises when the braking torque exerted on an axle exceeds a certain critical value, the axle locks and the vehicle skids, the driver often losing control of it in the process. It is therefore highly desirable to prevent such a condition, called "overbraking".
One well known manner of attacking this problem is to provide an automatic retaining device, for instance a hydropneumatic control valve, on the friction brakes of the axle on which the hydrodynamic brake is mounted. The retaining device reduces the braking torque exerted at any instant by the rear friction brakes, by an amount equal to the braking torque exerted by the hydrodynamic brake.